Red No. 40 and other artificial dyes have increasingly become scarlet letters (and numbers) to parents who read food labels. Some believe that these common food, drug and cosmetic dyes can exacerbate behavioral disorders in children, as well as possibly contribute to allergies and cancers.

The FDA, after long stating that no scientific evidence conclusively links the petroleum-based colorants to hyperactivity and other ills, has agreed to reassess them. A panel of experts will review studies and suggest any policy changes.

An outright ban on synthetic dyes is unlikely, though the nonprofit Center for Science in the Public Interest just petitioned the FDA to outlaw caramel colorings that use ammonium compounds, and in 2008 asked it to revoke approval for several synthetic food dyes.

At the time, it requested an interim warning label such as, "The artificial colorings in this food cause hyperactivity and behavioral problems in some children."

For a host of ills in children, pediatric specialists often prescribe diets that are free of synthetic dyes, among other additives. Few doctors single out dyes as the cause of a disorder. Still, the CSPI's site (cspinet.org/fooddyes) abounds with testimonials of tantrums tamed and focus improved when artificial colorants are avoided.

What all could agree on, for now, is that artificial dyes confer no direct nutritional benefits. So one might wonder: Why use them if you can lose them?

One big reason is cost. Artificial colors often are cheaper, brighter and more stable than natural plant-based colors, explained Bob Petrich, CEO of St. Paul, Minn.-based Suntava, which extracts a natural color from a proprietary purple corn. Another reason — whether a virtue or deception — is that added color can make not just junk foods but also nutritious ones, such as salmon, look more appetizing.

"Artificial colors are in nearly everything, including most prescription and over-the-counter drugs," said Shauna Young, a naturopath who works with children and adults with autism spectrum and attention disorders (NoHarmFoundation.org). "I find it particularly offensive that they are so prevalent in chewable vitamins aimed at kids."

In countries that have different dye regulations, food companies routinely substitute natural dyes for artificial ones. The CSPI cites Nutri-Grain bars. In Britain they contain natural colorants. In the U.S., synthetic.

Here, as suspicions have grown, more products at grocery chains — not just at Whole Foods and Trader Joe's — are using plant or vegetable colorants such as beet juice, turmeric, paprika, purple corn or annatto (from a tropical tree's seed).